This invention relates to catalysts used to remove undesirable components in the exhaust gas from internal combustion engines. More particularly, the invention is concerned with improved catalysts of the type generally referred to as three-way conversion or TWC catalysts.
The exhaust from internal combustion engines contains hydrocarbons. carbon monoxide and nitrogen oxides which must be removed to levels established by various government regulations. The aforementioned three-way catalysts are poly-functional in that they have the capability of substantially simultaneously catalysing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
Typical three-way catalysts which exhibit good catalytic activity and long life contain one or more platinum group metals (eg Pt, Pd, Rh, Ru and Ir) located upon a high surface area porous refractory oxide support, eg a high surface area alumina coating. The porous refractory oxide support is carried on a suitable non-porous refractory substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure or refractory particles such as spheres, pellets or short extruded segments of a suitable refractory material.
Three-way catalysts are currently formulated with complex washcoat compositions containing stabilised alumina, an oxygen storage component (primarily stabilised ceria) and precious metal catalytic components. The term xe2x80x9coxygen storage componentxe2x80x9d is used to designate a material which is capable of being oxidised during oxygen-rich (lean) cycles of the exhaust gas being treated and reduced during oxygen-poor (rich) cycles of the exhaust gas being treated.
Three-way catalysts typically have been based on platinum/rhodium catalysts in preference to palladium which suffered from certain disadvantages including the high sensitivity of palladium to poisoning by sulphur and lead. However, with increased use of lead free petrol around the world, palladium is an extremely promising substitute for the traditionally used platinum/rhodium catalysts. Furthermore, the much lower cost of palladium makes it a highly desirable alternative to platinum/rhodium in three-way catalysts, provided the desired catalytic performance can be achieved.
The art has devoted a great deal of effort in attempts to improve the efficiency of palladium containing three-way catalysts. Thus, in an article in Third Int. Cong. Catal. and Auto Poll. Controls, Pre-print Vol. 1, pages 125 to 135, the authors, Dettling et al, describe the inclusion of a low temperature catalyst component (Pd/Al2O3) and a high temperature catalyst component (Pd/CeO)2 in the same catalyst composition for high activity under both low and high operating temperatures.
WO 95/00235 (Engelhard Corporation) also describes a palladium containing catalyst composition containing low and high temperature catalyst components structured as two washcoat layers
WO 95/07600 (Allied Signal) describes a palladium containing three-way catalyst as a single layer. However, according to the method of preparation, the finished catalyst only has the high temperature Pd/CeO2 component.
U.S. Pat. Nos 4,727,052, 5,057,483, 5,008,090 and 5,010,051; GB Patent 1495637; and European Patent Applications 92302928.4 and 0427293A2 also describe three-way conversion catalysts based on platinum group metal catalytic components.
We have found that platinum group metal three-way catalysts containing a high temperature functional component and a low temperature functional component when prepared by the unique methods of the present invention exhibit greatly improved three-way catalytic activity even after extended high temperature aging.
In this specification, by high temperature functional catalytic component is meant a catalytic component which exhibits catalytic activity at higher temperatures (eg above about 500xc2x0 C.) and by low temperature functional catalytic component is meant a catalytic component which exhibits catalytic activity at lower temperatures (eg in the range 200 to 400xc2x0 C.)
According to the present invention there is provided a method of making a platinum group metal three-way catalyst composition which contains a high temperature catalytic component and a low temperature catalytic component with each catalytic component being present in the catalyst composition as separate distinct particles in the same washcoat layer, which method comprises:
(a) forming on a non-porous substrate a combined washcoat of a high temperature catalyst support material and a low temperature catalyst support material from a slurry in which each of the catalyst support materials is of sufficiently large particle size so as to prevent each catalyst support material from forming a solution or a sol with the liquid medium of the slurry; and
(b) impregnating a platinum group metal or metals into each catalyst support material either after formation of the washcoat on the non-porous substrate or before forming the washcoat slurry.
Preferably, separate slurries of the high temperature support material and the low temperature support material are prepared and the two slurries are then blended together and coated onto the non-porous substrate.
The non-porous substrate may be a refractory ceramic or metal honeycomb structure or refractory particles such as spheres, pellets or short extruded segments of a suitable refractory material.
Further according to the present invention, the proportions of the high temperature catalytic component and the low temperature catalytic component required in the catalyst composition are determined by the respective water absorption capabilities of each catalyst support material and the respective amounts of each catalyst support material present in the washcoat.
Preferably, the water absorption capabilities of the high temperature catalyst support material and the low temperature catalyst support material are respectively 0.2 to 1.0 ml/g and 0.5 to 2.5 ml/g.
Suitably, the catalyst support materials have a mean particle size of less than 20 microns, preferably between 1 and 20 microns and more preferably about 5 microns.
The platinum group metal is selected from platinum, palladium, rhodium, ruthenium, iridium or any combination thereof.
Preferably, the high temperature catalyst support material is an oxygen storage material.
Suitable oxygen storage materials include ceria, perovskites. NiO, MnO2 and Pr2O3 with stablised ceria being the preferred material.
Suitable stabilisers for ceria include zirconium, lanthanum, alumina, yttrium, praeseodymium and neodymium with zirconium being preferred.
Suitably, the zirconium stablised ceria contains 2 to 50% ZrO2, a preferred composition being about 58% by weight CeO2 and about 42% by weight ZrO2.
Suitable low temperature catalyst support materials are stabilised alumina and unstabilised alumina.
Suitable stabilisers for alumina include lanthanum, barium and zirconium with lanthanum being preferred.
Preferably, the lanthanum stabilised alumina contains 2 to 7% lanthanum oxide.
The method of the invention may utilise a catalyst promoter, preferably selected from Nd, Ba, Ce, La, Pr, Mg, Ca and Sr with Nd and Ba being particularly suitable. The catalyst promoters may be added to the slurry or separately impregnated.
Further preferably, the method of the invention utilises a compound effective for the suppression of hydrogen sulphide emissions from the catalyst composition. Suitable such compounds include NiO, Fe2O3 and BaO with NiO being preferred.
Suitably, the method according to the invention utilises a compound which is effective in preventing preferential absorption of the platinum group metal in one or other of the high temperature or low temperature catalyst support materials. Preferred such compounds include citric acid, acetic acid and oxalic acid.
From another aspect, the present invention is a platinum group metal three-way catalyst composition made by any of the methods described above.
From yet another aspect the present invention is a platinum group metal three-way catalyst composition comprising a high temperature catalytic component and a low temperature catalytic component wherein each catalytic component is present in the catalyst composition as separate distinct particles in the same washcoat layer.
Suitably, the high temperature and low temperature catalytic components in the catalyst composition have a mean particle size of less than 20 microns, preferably between 1 and 20 microns and more preferably about 5 microns.
As can be seen from the foregoing discussion of the prior art, the concept of combining a high temperature catalytic component and a low temperature catalytic component in the same three-way conversion catalyst is known. The present invention however, enables both catalytic components to be advantageously incorporated into a single washcoat layer by utilising a unique preparation technique. This preparation technique entails incorporating two distinct and separate catalyst support materials into the same washcoat slurry so that the final catalyst composition has both the high temperature catalytic function and the low temperature catalytic function in a single washcoat layer.
A key feature of the invention is that the catalyst support materials should not be in solution in the washcoat slurry or present as very small particles as found in a sol (the order of magnitude of the size of sol particles being in the nanometer range). In order to obtain the benefits of the present invention, the insoluble catalyst support materials in the washcoat slurry preferably should have a mean particle size of at least 1 micron, more preferably about 5 microns. However, if the particle size is too large (eg greater than 20 microns) there may be difficulty in getting the washcoat to adhere to a non-porous substrate.
Another important feature of the invention is that to maintain separation of the catalyst support materials they should be ball-milled in separate slurries followed by blending of these slurries. The final blend is coated onto the non-porous substrate.
Yet another important feature of the invention is the incipient wetness water absorption capabilities of the high temperature catalyst support material and the low temperature catalyst support material because these water absorption capabilities relate not only to the process for making the catalyst composition but also to the specification of the catalyst formulation. The catalyst contains two oxide support materials, exemplified by zirconium-stablilised ceria and lanthanum-stabilised alumina, although unstabilised alumina may be used. The platinum group metal (exemplified by palladium) is split between the two oxide support materials. In one embodiment of the invention, the palladium is impregnated from an aqueous solution into the washcoat consisting of a mixture of the two oxide support materials and the way in which the palladium is split between the two oxides depends on the fraction of the aqueous impregnation solution absorbed by the respective oxides. For example, if it is required that 50% of the available palladium is to be supported on the zirconium-stabilised ceria and the other 50% of available palladium is to be supported on the lanthanum-stabilised alumina then the washcoat would be formulated so that the water absorption of the zirconium-stabilised ceria in the catalyst composition (ie (ml water absorbed/g)xc3x97(g in catalyst)) is equal to the water absorption of the lanthanum-stabilised alumina in the catalyst composition. Thus, the ratio of the oxide support materials is specified by their relative water absorptions and the absolute amounts of the oxide support materials is specified by the amount of support needed in the catalyst composition (more specifically, a certain amount of Zr-stabilised ceria is needed for adequate performance). The desired split of the palladium depends on the duty required of the catalyst composition. In some applications, equal amounts of high temperature catalytic component and low temperature catalytic component is required. In other applications, more high temperature compound than low temperature compound is required (or vice versa). For example, catalyst compositions having palladium splits ranging from (a) 27% of Pd as Pd/ZrCeO2xe2x80x9473% of Pd as Pd/La Al2O3 to (b) 73% of Pd as Pd/ZrCeO2xe2x80x9427% of Pd as Pd/La Al2O3 have been prepared according to the methods of the invention.
In an alternative method of making the catalyst composition, a portion of the total palladium is impregnated into a bulk form of the high temperature catalyst support material and the remaining portion of the palladium is impregnated into a bulk form of the low temperature catalyst support material prior to the formation of the washcoat slurry. Since the impregnated palladium is essentially insoluble in the washcoat it remains interacted with its associated oxide support material in the final catalyst composition. In this embodiment also, the ratio of the two oxide support materials is chosen on the basis of their relative water absorptions and the desired split between the palladium intimately interactive with, for example, the zirconium stabilised ceria and the palladium intimately interactive with the lanthaniun stabilised alumina.